<?php
function sub($a,$b){
  return $a-$b;
}
  function get($name){
    return isset($_GET[$name])? $_GET[$name]:'';
   }
   function post($name){
     return isset($_POST[$name])? $_POST[$name]:'';
    }
  $a = post('a');
  $b = post('b');
  $result = sub($a,$b);
  ?>
<form action="" method="post">
<input type="text" value="<?php echo $a; ?>" name="a"/>-
<input type="text" value="<?php echo $b; ?>" name="b"/>=<?php echo $result;?><br>
<input type="submit"/>
</form>